Repair Stud or Helicoil?

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  • sglazebrook
    Senior Member
    • Mar 2013
    • 75

    Repair Stud or Helicoil?

    It's time to replace another manifold stud…

    Previously when installing a new MMI manifold (about a year ago), the middle stud wouldn't tighten down. So, my mechanic drilled-out the block for a helicoil which worked great to hold the manifold in place.

    Now, to address a leaking exhaust flange and manifold gasket I've pulled it all apart, built a new hot section with MMI exhaust flange, and reassembled only to find the front stud not tightening (and a small water leak from the middle stud). So, time to replace the front stud.

    I've read a lot on here about the different methods of replacement, have helicoils and the MMI 7/16" replacement stud, and I'll be drilling out the block on Saturday. My question is, which way is best? Once you go down one route you cannot change (even though the 7/16" tap drill bit is smaller that the helicoil drill bit - 23/32" versus 25/32" - once the 7/16" threads are cut into the block you probably aren't going to get enough bite later to install the helicoils as a subsequent repair job). Are the helicoils more likely to allow water to pass? Are they more solid and durable since you aren't relying on my ability to properly tap the hole and the integrity of the block material just beyond where it has already failed (threads)? Is the JB weld necessary for the MMI repair stud?

    Thanks in advance for your sage advice!
    Last edited by sglazebrook; 07-28-2014, 11:39 AM.
    Scott
  • sastanley
    Afourian MVP
    • Sep 2008
    • 6986

    #2
    I have a repair stud in my rear manifold hole, so of course I would recommend that..it seems a little less invasive than a helicoil. If it was me, I would try the repair stud first, and then then resort to a helicoil in a few years after the block fails again and will no longer hold a repair stud.

    edit - The Moyer repair studs are just long enough to hold the manifold...make sure you measure the distance of the other studs..since these bottom out in the water jacket, it is easy to get the stud too far in the block, and then you don't have enough threads at the other end for the nut..I almost went too far, but was just able to get the nut fully threaded...Ken also recommends the wider shouldered head nuts for the manifold too, so when I had mine off and did the repair stud, I replaced all three nuts.
    Last edited by sastanley; 07-28-2014, 09:55 AM.
    -Shawn
    "Holiday" - '89 Alura 35 #109
    "Twice Around" - '77 C-30, #511 with original A-4 & MMI manifold - SOLD! (no longer a two boat owner!!)
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    • sglazebrook
      Senior Member
      • Mar 2013
      • 75

      #3
      Just installed the MMI repair stud. It was about 1/4" longer than the standard manifold stud - which presented a strange problem.

      I didn't need to drill-out the old stud hole in the block - just hand-turned the correct drill bit ("U") in the hole to clean out the gunk and ran the taps to create the new thread. Threaded the repair stud in and tried to put the manifold on and found that the repair stud was at quite an angle towards the front of the engine and couldn't get all three studs into the manifold! I can't imagine I "drilled" the hole crooked or threaded it crooked (at least not that much). I'm guessing the hole was always a bit off but with a shorter stud I could get the manifold on and off (although not with out a lot of effort!).

      So, not having a stable workplace to cut the stud shorter I inserted it "backward" into the manifold and slowly screwed it in as I put the manifold on over the other to studs. Took too much time but it worked and I was able to tighten the nut and cutoff the excess in-place and torqued the nuts to 25. Started it up and no leaks present!

      The only (other) odd thing is that when I took the manifold off and placed it aside with carburetor still attached, it leaned a bit "forward" towards the carb throat and the throat filled with a dark brown "oily" substance from inside the manifold (I presume). Mixed with gas from the bowl it just smelled like gas. I cleaned the carb two weeks ago which is why I'm thinking it was from the manifold. And, last week there was an oily sheen on the water when I stared up so I was thinking the mixture was to rich (clean carb). I was able to get another half-turn out from the idle screw to lean the mixture before the engine hesitated and today there was less sheen on the water and a little less visible exhaust when accelerating. Now idling at about 775 in neutral and 625 in gear.

      So, I think all is good! Next project is revise the wet exhaust routing and the cooling water bypass/loop.
      Scott

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      • hanleyclifford
        Afourian MVP
        • Mar 2010
        • 6990

        #4
        Manifold studs should be threaded into the block with Permatex #2 on the threads because the stud is sealing a water jacket. Keep an eye on that stud for coolant leak.

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        • sglazebrook
          Senior Member
          • Mar 2013
          • 75

          #5
          Thanks! Got the Permatex #2 in there...
          Scott

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